This invention relates to a radical power combiner for microwave signals and more particularly to resistors for suppressing undesired modes, which resistors are made from inks fired at high temperatures in order to provide good thermal bonds to their heat sinks.
Communications systems are making increasing use of Earth satellites as transponders. The use of satellites for communication links between cities eliminates the need for land communication cables, which are very costly. In order to provide continuous coverage, a satellite must be in a geosynchronous orbit. Such orbits require that the satellite be at an altitude of about 22,000 miles. Thus, communications by way of a geosynchronous satellite requires transmission over a path length of 22,000 miles to the satellite and transmission from the satellite over a 22,000 mile path length to the receiving Earth station. Transmission over such a distance requires relatively high antenna gain. The necessary gain is achievable with antennas of reasonable size and reasonable cost only at microwave frequencies and frequencies higher than microwave.
The transmission of signal from the satellite to the Earth station requires a power amplifier located in the satellite capable of generating tens or hundreds of watts of microwave power with great reliability. In the past, the microwave power was generated by travelling wave tubes (TWT). Travelling wave tubes were, and continue to be, used for satellite transmitters notwithstanding the reliability problem attributable to the inherent degregation resulting from operation over a period of time. More recently, solid state power amplifiers (SSPA) have been used at lower microwave frequencies, such as at C-band instead of travelling wave tubes. The SSPA has no inherent degradation mechanism so is more reliable than the TWT. A need exists to provide solid state power amplifiers at X-band (around 10 GHz) and at millimeter wave frequencies.
Solid state power amplifiers are implemented by using a large number of relatively low power solid state devices. Each solid state device provides a small portion of the total output power, and power combiners are used to combine the powers from each of the individual solid state devices to generate the desired amount of signal power at microwave or millimeter wave frequencies.
Various types of power combiners are described in the article "Microwave Power Combining Techniques" by Kenneth J. Russell, published in the IEEE Transaction Microwave Theory and Techniques, May 1979. In the Russell article, corporate or tree combiners are described as being useful for combining a small number of devices but as being very inefficient as the number of devices combined increases. Similarly, the chain type of combiner is not useful. Russell also describes resonant and nonresonant N-way combiners. Among the more successful techniques for combining power which he describes are the cavity combining technique. However, this technique has limited bandwidth. It is desirable to have a broad bandwidth in a satellite transmission channel in order to maximize the usefulness of the expensive satellites.
U.S. Pat. No. 4,291,278 issued Dec. 22, 1981, to Quine describes a power amplifier including a feed waveguide, a fin-line array transition from waveguide to microstrip, a plurality of amplifiers each of which is fed from microstrip, a plurality of phase shifters at the output of the amplifiers for compensating phase, and a fin-line array transition from microstrip to waveguide. This structure requires a phase compensator for each amplifier in order to compensate for the different path lengths from the common feed point to each amplifier, and therefore has the additional problem of requiring alignment of the phase compensators. Furthermore, each phase compensator presumably has a different loss and this results in combination of unequal powers. As the number of amplifiers increases from a few to a very large number, the linear dimensions of the Quine amplifier increase proportionally and it can be very large. Also, the length of the transmission lines to and from the amplifier most remote from the feed point tends to reduce the effectiveness of the structure in combining the power.
A power amplifier is desired which is easy to manufacture and suitable for use at microwave and millimeter wave frequencies, which has relatively small linear dimensions when large numbers of amplifier modules are used, in which each amplifier module is provided with positive heat sinking, and each module can be accessed for maintenance without substantial disassembly of the structure.
U.S. Pat. No. 4,263,568 issued Apr. 21, 1981, to Nemit describes a radial power splitter/combiner which provides low loss N-way power splitting and/or combining. The Nemit structure includes a radial parallel plate waveguide which is coupled at its center to a coaxial common port. At the periphery of the radial parallel plate waveguide, a number of wedge-shaped loops sample the energy. Because of the symmetry of the structure, each loop receives an equal amount of the energy from the common port in splitter or divider operation. When used as a combiner, application of equal energies to the N ports causes their sum to appear at the common port, less any losses. Unavoidable asymmetry due to construction tolerances causes unequal power to appear at each of the N ports when operated as a splitter. These asymmetries, or inequality of the powers applied to the N ports when operated as a combiner, may lead to an undesirable operating mode in which energy flows around the periphery of the structure. This mode may cause high VSWR, excess insertion loss and/or phase shifts. In order to reduce such undesirable effects, the symmetry of the structure should be as good as possible, and when operated as a power combiner, the signals applied to the N ports should be applied in equal amplitude from sources having equal source impedances. Any residual circulating energy may be attenuated by resistors coupled across the slots between coupling loops.
At frequencies in the microwave and millimeter-wave regions, it is difficult to make a completely symmetrical N-way divider or combiner where N is large, because of the small physical size of the details of the structure. For this reason, it is very advantageous to draw the circuit in a large size so that the symmetry and circuit details can be clearly defined, photographically reduce the drawing and reproduce it as a printed circuit board. The resulting printed circuit board does not depend on the skill of a machinist for symmetry and fine detail.
The printed circuit board is formed from an organic material having a low dielectric constant and low loss in order to allow a practical printed circuit board thickness and transmission line width. Chip resistors can be soldered to the printed circuit metalization. This is a laborious procedure which must be performed by skilled personnel, and is subject to inadvertent errors such as solder bridges located beneath the chip resistors which short-circuit across the gaps but are invisible to the eye. These short-circuits are especially difficult to locate with direct-current instruments such as multimeters since the gap is intentionally short-circuited at the end. Chip resistors soldered to the printed circuit metallization and coupled across slots formed in the metallization performed satisfactorily in a power dividing mode. When used as a power combiner to combine the power from 30 solid state amplifier modules, however, some of the resistors were destroyed by the application of power.
It would be desirable to have an arrangement for suppressing undesirable circulating modes in a radial power splitter/combiner which is easily fabricated, and rugged.